Microstructural Analysis of Spall Damage Nucleation and Growth in Multicrystalline Titanium

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Shock loading is a dynamic condition that can lead to material failure and deformation modes at the microstructural level such as cracking, void nucleation and growth, and spallation. Knowledge of shock loading and spall failure is of great benefit to understanding ballistic impact in military vehicles and armor, crash impacts in automobiles, space vehicles, and satellite loadings, and geological events such as earthquakes. Furthermore, studying material failure at the microstructural level is important to understand macroscale behavior. Spallation, the nucleation, growth, and coalescence of voids, is a phenomenon where variability at the microscale can affect overall response. By analyzing incipient and intermediate damage patterns at and around interfaces and boundaries on the microstructural level, can help further our understanding of the process leading to damage and provide insight on how to develop stronger structures that can withstand impacts and rapid crack propagation. Most of the existing work has looked into the effect of grain boundaries in spall damage for body and face centered cubic (BCC, FCC) materials, but research is still lacking on grain boundary effects in spall damage in hexagonal close packed materials, such as titanium. Samples of high purity Ti were heat treated to obtain large grains, averaging 250 microns in size (multicrystals), in order to isolate grain boundary effects. These multicrystals were shocked using laser-launched flyer plates at the Trident laser at Los Alamos National Laboratory (LANL) and monitored using a velocity interferometry system for any reflector (VISAR). Pressures used were 5–8 GPa. Samples were soft recovered and cross-sectioned to perform quantitative characterization of damage. Spallation damage observed in the titanium targets was characterized using electron backscattering diffraction (EBSD), optical microscopy, and scanning electron microscopy (SEM) to gather information on the crystallographic characteristics of damage nucleation sites, with emphasis on grain boundaries and grain orientations that lead to damage localization. Initial results show that damage localized along grain boundaries, and the damage mode switched from intergranular to transgranular where grains were larger than average.